Currently, optical data communication systems are being upgraded from a 10 Gb/s data transmission rate up to a 40 Gb/s transmission rate. However, data transmission at 40 Gb/s (or higher) presents extensive design challenges because optical fiber dispersion, including both polarization mode dispersion (PMD) and chromatic dispersion, and fiber non-linear effects, such as cross-phase modulation, become more dominant at the higher transmission rates. For example, the limit of tolerable polarization mode dispersion, usually defined as 14% of the data bit duration, is only 3.5 ps at a 40 Gb/s transmission rate. A 3.5 ps polarization mode dispersion translates to an attainable reach of several hundred kilometers over single mode fiber which has a typical fiber PMD of 0.1 ps/km1/2.
Commonly owned and assigned patent application Ser. No. 09/782,354 describes how side carriers transmitted with orthogonally polarized data bands occupying the same optical frequency band can be used to effectively separate the data streams in the orthogonally polarized data bands, providing for an increase in the amount of data that can be received within the frequency band, or, phrased alternatively, an increase in spectral efficiency. Furthermore, commonly owned and assigned patent application Ser. No. 09/871,216 describes a quadrature-return-to-zero modulation technique in which the power of a transmitted quadrature-modulated data signal drops to zero between data symbols, rendering the power of the data signal independent of data content. The techniques described in these applications, which are expressly incorporated by reference herein, provide intrinsic benefits in terms of spectral efficiency and nonlinear performance. These benefits can be harnessed and extended through techniques and systems that increase the data rate and spectral efficiency of an optical data communication system beyond 40 Gbps (gigabits per second) over a 100 GHz channel, and that also provide robust performance by further minimizing the dominant dispersion and nonlinear effects. Additionally, the techniques can also be extended to enhance the signal-to-noise and nonlinear performance of data transmission at any desired data rate.